Friday, 7 November 2014

ALMA Spots Planets Forming Around a Young Star

High-resolution image of the protoplanetary disc around HL Tau, a young star roughly 450 light-years from Earth. The image, which was taken by the ALMA telescope, shows gaps and rings in the disc carved out by new-born planets. Image Credit: ALMA (ESO/NAOJ/NRAO) 
This morning the image at the top of this page was doing the rounds on Twitter. I, like several others, glanced at it and initially moved on. I've seen plenty of artist's impressions like it before. It took me a while to realise that this isn't a painting. This is a real image from the ALMA telescope, showing the birth of a solar system.

The image shows a star surrounded by a protoplanetary disc, a huge ring of gas and dust around twice the diameter of Neptune's orbit. Invisible at the wavelengths of light that ALMA sees, the central star is a young object called HL Tau, which is  just a million years old. That might seem old, but our own Sun, which is otherwise quite similar to HL Tau, is 4.6 billion years old. HL Tau is a star at the very beginning of its life.

This makes the disc partly expected, but partly mysterious. For the past few decades most models of how planets form have been based on discs like these, the leftover debris from the cloud that collapsed to form the star.  HL Tau is making planets.

Although the entire process is till not fully understood, the theory suggests that slight irregularities in the disc can cause some areas to become more dense. This makes them clump together, growing from dust into small rocks. As they get larger their gravity gets stronger, pulling in more and more material until they begin to look like small planets or asteroids. These planetesimals begin to collide, combining to eventually form planets.  

The ALMA image is a resounding confirmation of this theory, showing this process in action. The disc has huge gaps in it, gaps which are carved out by newly-forming planets. This is the mysterious part, as the presence of these very well defined gaps, at such a young star, show that the planets must be growing much quicker than many simulations suggest.

Not all of the gaps will have planets in them. Some of them will be formed by resonances. This means that, for example, an area of the disc could be going round the star a certain, precise  number of times in the time it takes a further out planet to go round once.

For example, a dust particle in the disc could be going round the star four times for every time planet, which is further away form the star, goes round once. The planet and the dust will then be lined up at exactly the same place each time the planet goes around the star. The gravity of the planet will give the dust an identical tug or kick each time, moving it out of it's orbit.

As this will happen to all of the dust in the same resonant orbit, eventually a gap is cleared. We see the same behaviour in this Solar System- the many rings of Saturn are shaped and sculpted by moons in resonant orbits. Which of the rings in HL Tau are formed by planets, and which are cleared out orbital resonances, will take more observations to find. The full research paper on this observation is yet to be published, so maybe we'll find out then.

Hubble Space Telescope image of the clouds of star-forming gas and dust around HL Tau. Image Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA
HL Tau is in the constellation Taurus, currently visible in the late evening in the Eastern sky, near the Moon. But you wont spot the disc, it's far too small.

This leads to the second incredible part of this image: The resolution. The image was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), a huge array of 43 (and counting) telescopes designed observing in the submilliter wavelength range, between infrared and radio waves. The telescopes are spread out by up to fifteen kilometers, allowing them to take extremely sharp images.

ALMA can distinguish between objects separated on the sky by just 35 milliarcseconds. For comparison, your eye has a resolution of around one arcminute- nearly two thousand times worse. Resolution like ALMA would allow you to see both sides of a penny placed over one hundred kilometers away.

It's this high resolution that has allowed ALMA to see the protoplanetary disc in such exquisite detail, picking out the tracks formed by the new, growing planets. And this is in many ways just a proof of concept, a test of ALMA's capabilities. Hopefully we'll be seeing many more amazing things with this telescope in the future.

New blogs will be posted, as ever, on Twitter.

COMING SOON: Next Wednesday (12th) The Rosetta spacecraft will deploy the Philae lander to make the first attempt to land on a comet. I'll be tweeting and blogging along, and I highly recommend keeping track of, in my opinion, the most exciting space event of the year.

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